So, I drag the cluster, battery and 20A charger out to the deck to do a full load test. This is the first time I’ve fired this newly built controller on a full load. Uncharted territory so far.
Here’s the set up.
The charge controller that I built earlier is in a nice shiny re-purposed case that I had laying around. Just the right size too. These were USB extender devices that my father’s workplace had used in a project: they wanted the innards, so we got the empty boxes. I just mounted the PCB on a small piece of plastic to insulate it from the case, and routed my DC leads out through a hole in the case originally intended for an RJ-45 jack.
At this point, everything is humming along fine. Our battery charger is on stand-by.
The meter is showing a sedate 12.4V and the controller is happy with this. That said, I’ll have to work on the visibility of those LEDs. The two on the power MOSFET control lines are off at this stage.
So I give the system a bit of curry. I transfer a copy of a Linux kernel git repository to each, tell them to update their working copies from that, and build a version of kernel v4.8.5. This made the current jump up to about 10A. So far so good, the battery is holding.
Then, about 30 seconds in, the controller decides it’s a bit too low, so it kicks the charger on. There’s a few false starts, as the charger delays its start-up a bit. Eventually though they get into sync and start charging. At this point, the charger is taking the load of the battery and the cluster.
Great. So it continues for a minute, then decides it wants to shut down, which it does, followed by a moment of oscillation. It seems the controller is too impatient for the charger, waiting for the power to come on…but then… what’s that smell???
Oops, guess that MOSFET got just a little too hot. I was hoping to avoid the need for heatsinks by over-dimensioning the MOSFETs. These are supposedly able to take 70A, I realise that’s with a heatsink, but I thought that at 20A, they would be able to handle it.
One somewhat roasted MOSFET says otherwise. Interestingly, only one has visible marks, its mate looks okay, but likely isn’t.
The MOSFETs don’t have to be mounted directly on the PCB, we can re-locate them to where we can squeeze a heatsink in if I can’t get one in between the two already. The thought was each pair have a heatsink in between them. More pondering to do it seems.